Military officer, say (7)
I believe the answer is:
martial
'military' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for martial that I've seen before include "Soldierly (arts?)" , "Specialist in epigrams" , "relating to soldiers" , "Relating to the armed forces" , "Relating to hostilities" .)